Usability...What it is?

Came to know about a new field of design(wonder if its new....but yes it is for me ;-)) called 'USABILITY'. Just trying to know something about this. Went through some links from a senior's blog. This basically deals with the user experience with the application/website whatever it is and would need a gud understandign about the users of the respective scenarios. Its basically making the web more usable. Am exploring more about this. Any comments would be appreciated...